Ion beam scanner system and operating method

ABSTRACT

The invention relates to an ion beam scanning system having an ion source device, an ion acceleration system and an ion beam guidance system comprising an ion beam outlet window for a converging centered ion beam, and a mechanical alignment system for the target volume to be scanned. For that purpose, the ion acceleration system can be set to an acceleration of the ions required to obtain a maximum depth of penetration. The scanning system also has energy absorption means arranged in the path of the ion beam between the target volume and the ion beam outlet window transverse to the center of the ion beam. The energy absorption means can be displaced transverse to the center of the ion beam in order to vary the energy of the ion beam, so enabling, in the target volume, depth modulation of the ion beam, which is effected by means of a linear motor and the transverse displacement of the energy absorption means, with depth-staggered scanning of volume elements of the target volume in rapid succession. The invention relates also to a method of ion beam scanning and a method of operating an ion beam scanning system using a gantry system.

The invention relates to an ion beam scanning system and to a method ofoperating the system according to the preamble of claims 1 and 20.

Such a system is known from Specification U.S. Pat. No. 5,585,642 and isused in particle therapy. The ion beam therapy that can be carried outon tumour tissue using such a system is distinguished primarily by abetter dose distribution, that is to say a higher tumour dose, and thefact that it reduces the radiation to which healthy tissue is subjected,compared with X-ray therapy. That dose distribution is the result of thephysical properties of particle beams, which have an inverse doseprofile, that is to say the dose increases with the depth ofpenetration. As a result, the tumour dose can be increased compared withthe conventionally possible dose obtained by customary radiationtherapy.

In order to obtain as good an adaptation as possible of the irradiatedvolume to the predetermined target volume, in current clinical practicethere are used devices for passive beam formation, which are not,however, able to solve the problem satisfactorily. Such beam formationdevices work with a divergent ion beam that irradiates a larger areathan the target volume, that is to say than the volume of the tumour,but restrict the divergent ion beam to the tumour volume by appropriateedge-delimitation devices and by the compensation shapes made ofcompensation materials, which are modelled on the outline of the tumour.Such systems and methods have the disadvantage that a high ion beamenergy is required for the divergent ion beam and it is not possible totarget specifically individual volume elements of a target volume ortumour.

In order to be able to target individual volume elements specificallyand to be able to adapt a radiation dose optimally for the volumeelement, a raster scanning device for ion beams was developed. By meansof that device, the target volume is broken down into layers ofidentical particle range and a fine, intensity-controlled pencil beam ofions is guided over the individual layers in the form of a raster.Together with the active energy variation by means of an ionaccelerator, it is thus possible to achieve precise illumination inthree dimensions of any desired target volume.

That intensity-controlled raster scanning device, however, also hasconsiderable disadvantages. Firstly, a complicated control system formonitoring the beam localisation in the microsecond range is necessary.There is also the risk of fragmentation of isoenergy steps as a resultof non-homogeneities of density. It is also problematic to maintain thepredetermined lateral beam position (focal point of the beam),especially in the event of variation in the energy and beam width duringtreatment. Finally, the edge cut-offs of the target volume, which aredependent on the width of the beam profile, are disadvantageous forprecise irradiation of the target volume.

Those problems mean that the adjustment and control of the beamparameters of such a device take much more time than does the actualpatient irradiation. Moreover, the combination of raster scanning devicefor ion beams and a movable rotatable ion beam guidance system of U.S.Pat. No. 5,585,642, a gantry system, as is proposed in expert circles,represents a considerable technical challenge.

In the case of incorporation of the raster scan in a gantry system, asis known from Specification EP 0 779 081 A2, the control procedures areeven more time-consuming and complicated than in rigid beam guidanceachieved hitherto. Moreover, magnets having large apertures are requiredfor incorporation of the proposed raster scanning device in order toachieve practicable sizes in the irradiation fields. Combined with arapid variation in energy, the large apertures of that proposed solutionand thus the required screening-off of scatter fields excludes the useof superconducting magnets. Incorporation of a scanner system thusresults in large apertures, that is to say in large deflection magnetsand long gantry systems that take up a lot of space and are expensive.In an arrangement of the scanner system behind the last deflectionmagnet, that is to say downstream of the ion beam guidance system anddownstream of an outlet window for the ion beam from the guidancesystem, as is known from Specification EP 0 779 081 A2, small aperturesare possible, that is to say compact magnets can be used, but result ingantry radii of more than 7 m because of the necessary drift length orthe internal width for a treatment room. Accordingly, disadvantageously,in both cases masses of more than 100 tons have to be moved withmillimeter-precision accuracy.

The problem of the invention is to overcome the disadvantages of theprior art and especially, compared with the proposed ion beam rasterscanning device, to provide an ion beam scanning system that overcomesthe current difficulties.

That problem is solved by the subject matter of claims 1 and 20. Furtherfeatures of preferred embodiments of the invention are given in thedependent claims.

In order to solve the problem, there is provided an ion beam scanningsystem having an ion source device, an ion acceleration system and anion beam guidance system comprising an ion beam outlet window for aconverging centred ion beam, to which there is connected a mechanicalalignment system for the target volume to be scanned. Such alignmentsystems are known from the prior art and are intended to ensure that thetarget volume can be irradiated from any determinable angle in the room.Accordingly, such an alignment system customarily comprises apatient-carrying table that can be rotated about at least one axis ofrotation and that can be moved in three displacement directions which,after alignment, customarily (in some cases) are not further alteredduring irradiation.

In the ion beam scanning system according to the invention, the ionacceleration system can be set to an acceleration of the ions requiredto obtain a maximum depth of penetration, and the scanning systemcomprises energy absorption means that are arranged in the path of theion beam between the target volume and the ion beam outlet windowtransverse to the centre of the ion beam and can be displaced transverseto the centre of the ion beam in order to vary the energy of the ionbeam. In order to modulate the depth of the ion beam, according to theinvention for that purpose the energy absorption means are displacedtransverse to the ion beam by means of a linear motor, it advantageouslybeing possible to carry out, in the target volume, depth-staggeredscanning of volume elements of the target volume in rapid succession.

That ion beam scanning system, which is based on depth modulation,offers an advantageous improvement over the installation andincorporation of a raster scanning system into a gantry system. Thesystem according to the invention enables dose distribution that is asgood as that achieved with the raster scanning system, but requires asubstantially less complicated control system. Moreover, it permitscompact construction even in conjunction with superconducting magnets,so that with that system it is possible to achieve any desired fieldsizes. Finally, with that system the healthy tissue above a tumourtissue is spared to a greater extent and in particular the skin isspared better. Moreover, the ion beam scanning system according to theinvention can be used universally both for rigid ion beam guidancesystems and for rotatable ion beam guidance systems.

In a preferred embodiment of the ion beam scanning system, the energyabsorption means have absorber wedges that can be displaced transverseto the centre of the ion beam, which absorber wedges are driven by ahigh-performance linear motor, so that beam-intensity-controlleddepth-scanning can be carried out. The absorber wedge system modulatesthe depth of penetration of the beam by means of retardation, that is tosay the Bragg maximum is modulated over the tumour depth. Lateralmovement can be effected by displacing the patient in two directions,for example in the x- and y-direction of a plane. That has the advantagethat only a fine pencil beam of fixed energy has to be controlled by theoverall ion beam guidance system. The fixed position of the beam in thatsystem can advantageously be guaranteed by a mechanically fixed aperturediaphragm and can also be checked by small locally resolving counters.The beam intensity can be measured by a simple ionisation chamber inorder to add up the beam dose per volume element.

In a preferred embodiment, the scanning system comprises an electroniccontrol system for the linear drive of the absorber wedges and anionisation chamber for measuring the particle rate of the beam. Theabsorber wedges are moved closer together by a step, preferably of from10 to 100 μm, when a predetermined particle count has been reached,which particle count is measured by the ionisation chamber and may becompletely different for the depth step, so enabling depth-staggeredscanning of volume elements of the target volume.

That embodiment has the advantage that large locally resolvingdetectors, which check the total radiation field, are not required. Thatmeans that the control system can be reduced considerably and the systemas a whole is smaller.

Theoretically, the absorber wedge system can consist of a singleabsorber wedge which is moved step-wise transverse to the ion beam andwhich, on account of its increasing thickness, reduces the depth ofpenetration of the ion beam into the tissue or target volume. Thisresults in scanning of the target volume in columns. An absorber wedgesystem that has only one absorber wedge has the disadvantage, however,that the absorption, and therefore also the depth of penetration, variesover the width of the ion beam. In a preferred embodiment of theinvention, accordingly, the energy absorption means comprise at leasttwo absorber wedges that can be displaced in opposite directionstransverse to the centre of the ion beam. Those two absorber wedges havethe same absorber wedge angle so that when the two absorber wedges aremoved together stepwise, the ion beam must always pass through the samethickness of an absorber material. Even in this case, however, minimalenergy differences arise over the cross-section of the ion beam sincethe ion beam itself converges and thus has a different cross-section atthe absorber wedge faces as it moves from one absorber wedge to thesecond absorber wedge and is thus absorbed differently over thecross-section.

In order to reduce that effect, the energy absorption means preferablycomprise two absorber wedge assemblies that can be displaced in oppositedirections transverse to the centre of the ion beam. In such absorberwedge assemblies, the gap between two absorber wedges is distributedover a plurality of gaps in which the above-mentioned disadvantageouseffect is obviated to a great extent by appropriate arrangement of theabsorber wedges relative to one another and the gradient per wedge isreduced compared with a system having two wedges that can be displacedin opposite directions to each other.

In a further preferred embodiment of the invention, the ion beamscanning system comprises an edge-delimitation device havingdisplaceable shutter elements between the target volume and the energyabsorption means. Such mechanical edge-delimitation has the advantagethat steeper edge cut-offs become possible without a complicated controlsystem becoming necessary. For that purpose, the scanning systempreferably comprises edge shutters that can be adjusted separately inthe manner of an iris diaphragm for such edge-delimitation of the ionbeam with respect to the target volume.

In order to irradiate preferably the whole target volume, the ion beamscanning system comprises a patient table that carries the target volumeand can be displaced in a plane transverse to the ion beam in twodirections of co-ordinates during an irradiation procedure.

By means of the depth modulation system according to the invention, thetarget volume is firstly broken down into columns about the individualtarget beams. Along the column the path of the beam is divided intoindividual positions or pixels, for which the beam coverage or beam dosehas been pre-calculated. Using a mechanical retardation system for theenergy absorption of the ions, which consists of a multiple wedge with alinear drive, the Bragg maximum of the beam is guided in anintensity-controlled manner without interruption from one pixel to thenext pixel when the particle coverage of the individual pixels has beenreached. That division into columns corresponds better to the actualcircumstances of tumour irradiation than the division into flat surfacesof the proposed raster scanning device, since non-homogeneities indensity upstream of the target volume, for example in the healthy tissuestructure above a tumour, result in a displacement of a column in apositive or negative direction but do not result in an interruptionwithin the column.

For the treatment of a tumour in a patient, in the solution according tothe invention advantageously the highest energy required for the beamguidance system is set from the accelerator to the patient. That energysetting advantageously remains constant for the whole treatment sincethe depth variation, that is to say the energy variation, occurs only asa result of the rapidly movable absorber wedge system of the energyabsorption means directly in front of the patient. The length of theindividual irradiation columns depends upon the geometry of the targetvolume. The dose cross-section of the columns is a Gaussian profile. Thespacings between the centres of the columns must be smaller than halfthe half-value width of the Gaussian distribution in order to producecontinuous overlapping. In order to eliminate over-irradiated orunder-irradiated positions, a relatively broad beam profile in theregion of an order of magnitude of 10 mm in diameter is advantageous.Such beam profiles, which are large compared with those in the proposedraster scanning device, also reduce the duration of irradiation perpatient in the case of large target volumes.

Since some of the radial dose distribution of the individual dosecolumns can be absorbed by the edge-delimitation device of a preferredembodiment of the invention, it is possible to obtain a steep edgecut-off at critical positions despite large half-value widths.

Whereas the energy absorption means in the form of a depth modulationdevice, or a depth modulator or depth scanner, can scan the targetvolume rapidly and in columns because of the drive of the absorber wedgesystem by means of a linear motor having an air bearing, there issufficient time for lateral displacement of the target volume in the twodirections x and y of a plane, so that a patient table, which carriesthe target volume and can be displaced in two lateral directionstransverse to the ion beam during an irradiation procedure, hassufficient time to scan gradually column by column and overlap next toone another.

In a further preferred embodiment of the invention, the patient tablecan be displaced during treatment in only one direction of co-ordinates,whilst for the other lateral direction suitable deflection magnets forthe ion beam are provided so that the ion beam can be deflected from itscentral position at the outlet window transverse to the lateraldirection of the patient table. An advantage of that system is that itis dependent upon mechanical and thus slow movement in only onedirection, and depth modulation and lateral modulation can be carriedout relatively swiftly.

Preferably an ionisation chamber for summation of the ions that strike avolume element is arranged upstream of the energy absorption means anddownstream of the ion beam outlet window. By means of that arrangementit is possible advantageously to determine the radiation dose, which isdefined as the total number of ions that strike a volume element.

In a further preferred embodiment of the invention, the ion beamscanning system comprises, in addition to a patient table that can bedisplaced in a lateral direction, a gantry system that can be rotatedabout a gantry axis of rotation transverse to the lateral direction ofmovement of the patient table. Owing to the slow back and forth movementof the gantry system over the irradiation field, the individual columnsare positioned adjacent to one another and thus advantageously thesecond dimension of the tumour-conforming irradiation is effected.

In the preferred ion beam scanning system having a gantry system, theion beam is supplied to the gantry system in the gantry axis of rotationand aligned with a target volume by means of magneto-optics atadjustable angles of from 0 to 360° in a plane orthogonal to the gantryaxis of rotation. The ion beam thus intersects the gantry axis ofrotation at an isocentre of the gantry system. The gantry systemcomprises a target volume carrier that can be displaced laterally in thedirection of the gantry axis of rotation, which target volume carrier isarranged upstream of the isocentre. The energy absorption means arearranged radially upstream of the gantry system. Volume element scanningin the depth direction is achieved by means of the depth modulationdevice according to the invention, angular volume element scanning inthe lateral direction is defined by means of the gantry system andvolume element scanning in the longitudinal direction is defined bymeans of the laterally displaceable target volume carrier, with theresult that target volumes of any shape can be scanned by volume elementby a combination of those three scanning means.

In that preferred embodiment of the invention, it is important that theisocentre of the gantry movement lies downstream of, that is to saybehind, the irradiation volume. A slight conical shaping of theirradiation volume can be taken into account by appropriate weighting ofthe pixels in a dense network of radial support points, without loss ofhomogeneity.

The angular difference of the eccentric irradiation as a result ofgantry rotation has the decisive advantage of further reducing the doseload in the inlet channel and thus reducing the dose to which thehealthy tissue above the tumour tissue is subjected. Since, owing to itslarge mass, the gantry system can move only continuously or in smallsteps, the particle fluence supplied must be higher than the fluencerequired per column. That means that the irradiation of each individualcolumn is carried out in the course of a short angular movement of thegantry system. The third co-ordinate of the irradiation is effected, inthat preferred embodiment, by a slow stepwise displacement of therecumbent patient on the target volume carrier through the irradiationapparatus in the direction of the gantry axis of rotation. In thatprocedure, preferably speeds of less than 1 cm/s are observed and aresufficient for tolerable periods of irradiation of patients.

In a further preferred embodiment of the invention, the target volumecarrier remains stationary during treatment and the ion beam isdeflected in the gantry plane by the deflection magnets duringirradiation. Thus, instead of displacing the patient couch, the ion beamis deflected by varying the magnetic field in the last deflection magnetin the gantry. This results advantageously in the following requiredmovement sequence in three degrees of freedom: the beam deflection withthe highest speed occurs by means of the energy absorption means, ordepth modulator. At a medium speed (e.g. 4 mm every 1 to 2 s), the beamis guided in the gantry plane by the deflection magnets to the nextcolumn. The slowest movement is the rotation of the gantry, which iscarried out, after the irradiation of a row of columns, by rotating thegantry system to the next row of columns. The advantage of that ion beamscanning system is that the patient does not have to be moved and thegantry system does not have to move back and forth during irradiationbut can be rotated stepwise.

Compared with traditional systems, the variable deflection of the beamin the gantry plane requires only relatively little extra expense sincethe deflection magnets are necessary anyway. The change in thedeflection of the beam takes places slowly (at a rate of seconds) andthus also no high demands are made on the magnet power supply or on thecontrol system. A simple wire chamber, which must be selected to have arelatively low repetition rate, is sufficient as monitoring device forthe deflection variation of the beam. The control electronics for thatslow scanning technique of the present invention are much lesscomplicated than the proposed rival rapid raster scanning technique inwhich the beam positions are to be measured every 100 μs in order to beable to track and control the beam.

In a preferred embodiment of the ion beam scanning system having agantry system, the energy absorption means comprise absorber elementsthat can be displaced tangential to the circle of rotation of the gantrysystem. That embodiment is achieved in that the energy absorption meansare fixed directly to the gantry system, namely downstream of the outletwindow of the ion beam. In that embodiment of the ion beam absorptionsystem having a gantry system, it is also possible for there to bearranged, instead of one absorber wedge, at least two absorber wedgesthat can be displaced in opposite directions tangential to the circle ofrotation of the gantry system or radially staggered absorber wedgeassemblies that can be displaced tangential to the circle of rotation ofthe gantry system.

In a further preferred embodiment of the ion beam scanning system havinga gantry system, a central region of the target volume is arrangedupstream of the isocentre by at least ⅕ of the radius of the gantrysystem, so that the target volume itself does not lie in the isocentre.The advantages of that embodiment have already been described in detailabove. It should be emphasised that an optimal spacing between thetarget volume and the isocentre can be set in order for the radiation towhich healthy tissue is subjected to be kept low in the inlet channel ofthe irradiation.

In the preferred method of ion beam scanning using an ion source device,an ion acceleration system and an ion beam guidance system comprising anion beam outlet window for a converging centred ion beam, and amechanical alignment system for the target volume to be scanned, thefollowing method steps are carried out:

-   -   setting of the ion acceleration system to an acceleration of the        ions required to obtain a maximum depth of penetration;    -   detection of the ion beam intensity;    -   transverse displacement of energy absorption means of variable        thickness for depth modulation of the ion beam;    -   summation of the radiation ions of a volume element of a target        volume up to a predetermined radiation dose;    -   alteration of the depth of penetration of the ion beam by means        of transverse displacement of the energy absorption means when        the predetermined radiation dose of the volume element has been        reached in order to irradiate the next upstream volume element.

An advantage of that method is that the acceleration of the ions in theion acceleration system needs to be determined only once and can bemaintained during the entire treatment phase. The depth modulation ofthe irradiation is carried out exclusively by energy absorption meansarranged upstream of the target volume to be irradiated and downstreamof the ion beam outlet window. Since, apart from the energy absorptionmeans or depth modulator, there is no further material in the path ofthe beam before it reaches the tissue, the nuclear fragmentation isminimal and independent of the depth of penetration, since the totalabsorption caused by the depth modulator plus the depth of the tissueremains constant. A constant Bragg profile is thus achieved over thetarget volume depth. The use of special filters, also ripple filters, torender the Bragg profile uniform, as are used in conventional methods,is no longer necessary in the method according to the invention.

The movement of the absorber wedges for the depth modulation iscontrolled by the intensity of the incoming beam according to thecalculated model of the dose distribution. The length of eachirradiation column is divided into individual image points and the beam,that is to say the Bragg maximum of the beam, is advantageously shiftedfrom one pixel, or image point, to the next when the required particlecount has been reached. The method according to the invention thusoffers the greatest possible safety for the patient and a high degree ofprecision in the irradiation of tumour tissues and minimal radiation ofupstream healthy tissue.

In a preferred way of carrying out the method, an electronic controlsystem for the linear drive of the absorber wedges moves the wedgescloser together by a step after it has measured the particle rate of thebeam by means of an ionisation chamber and after a predetermined numberof particles, which may be different for each depth step, has beenreached. Depth-staggered scanning of volume elements of the targetvolume is thus effected. Preferably the width of the step by which theabsorber wedges are moved closer together is from 10 to 100 μm.

The measured intensity per volume element is from 10⁶ to 10⁸ absorbedions during scanning of the target volume. The target volume can bescanned continuously progressively in that, during the depth modulation,either the patient table or the gantry or both are moved simultaneouslyin the other two directions. Whereas scanning of the target volume inthe depth direction is always effected in columns as a result of thedepth modulation, those columns can be shifted in a zig-zag manner inthe case of continuously progressive scanning.

In a different preferred way of carrying out the method, the scanning ofthe target volume proceeds stepwise. That stepwise procedure isespecially advantageous when the movements are effected at differentspeeds owing to different masses to be moved.

Accordingly, preferably scanning of the target volume can be carried outcontinuously in the depth direction and stepwise in the lateral andlongitudinal directions, or scanning of the target volume can be carriedout continuously in the depth direction and in the lateral direction andstepwise in the longitudinal direction. In a preferred way of carryingout the method, the ion beam scanning system is operated using a gantrysystem. For that purpose, the following method steps are carried out:

-   -   1. arrangement of the target volume upstream of the isocentre;    -   2. scanning of the volume element in the depth direction by        means of energy absorption means arranged radially upstream of        the gantry system;    -   3. scanning of the volume element in the lateral direction by        altering the angle of rotation of the gantry system; and    -   4. scanning of the volume element in the longitudinal direction        by raster displacement of the target volume carrier.

An advantage of that method is that the target volume carrier or thepatient table has to be moved in only one direction during irradiation,whereas one of the other two directions is obtained by depth modulationby way of the energy absorption means and the other is carried out bythe rotational or back and forth movement of the gantry system.

Whilst for the displacement of the absorber wedge system of the energyabsorption means relatively small masses are to be moved using a linearmotor, the rotational movement of a gantry system is effectedcorrespondingly more slowly, whereas displacement of a patient table,owing to its lesser mass compared with a gantry system but greater masscompared with the energy absorption means, with its inertia lies betweenthose two devices.

In a further preferred method, the patient can lie completely at rest onthe patient table when a gantry system is used in which a target volumecarrier is aligned before treatment and remains stationary duringirradiation and the ion beam is deflected in the gantry plane by meansof the last gantry deflection magnets in order to carry out volumeelement scanning in the longitudinal direction.

Further advantages, features and possible applications of the inventionwill now be explained in more detail with reference to embodiments.

FIG. 1 is a diagrammatic representation of a first embodiment of theinvention.

FIG. 2 shows the principle of the overlaying of depth dose profilesdisplaced relative to one another.

FIG. 3 shows the principle of the scanning of a target volume in columnsusing the embodiment according to FIG. 1.

FIG. 4 is a perspective view of energy absorption means in the form of asingle absorber wedge.

FIG. 5 is a cross-section through energy absorption means having twoabsorber wedges.

FIG. 6 is a cross-section through energy absorption means havingabsorber wedge assemblies each comprising five absorber wedges.

FIG. 7 shows an edge-delimitation device in the inoperative position.

FIG. 8 shows an edge-delimitation device having two shutters in theoperating position.

FIG. 9 is a schematic diagram of a linear motor having an air bearing.

FIG. 10 is a schematic diagram of a second embodiment of the invention.

FIG. 11 is a schematic diagram of the arrangement above the isocentre ofa target volume in the embodiment according to FIG. 10.

FIG. 12 is a schematic diagram of a third embodiment of the invention.

FIG. 1 is a diagrammatic representation of a first embodiment of an ionbeam scanning system that comprises an ion source device (not shown), anion acceleration system (not shown) and a beam guidance system 1comprising an ion beam outlet window 2 for a converging centred ion beam3. The ion beam scanning system includes a mechanical alignment system(not shown) for the target volume 5 to be scanned.

The alignment system enables the target volume to be rotated at leastabout one axis and to be displaced in the direction of three spatialcoordinates so that the target volume can be irradiated by the ion beamfrom any desired selectable angle in the room. Such alignment iseffected before the actual irradiation of the target volume with ions.

By means of the ion acceleration system (not shown), firstly anacceleration of the ions required to obtain a maximum depth ofpenetration is set. That energy is not altered during the entiretreatment and ensures that the ion beam can penetrate the tissue to thedeepest point of the target volume. By means of energy absorption means7, which are arranged in the path of the ion beam between the targetvolume 5 and the ion beam outlet window 2 transverse to the centre ofthe ion beam and can be displaced transverse to the centre of the ionbeam in the direction of arrow A in order to vary the ion beam energy,the Bragg maximum of the maximum depth of penetration is shifted tolower depths of penetration by increasing energy absorption. Thatprocedure can be carried out continuously or stepwise so that depthmodulation or depth scanning occurs, which will be referred tohereinafter also as depth scan. The change in the energy absorption isachieved by means of the energy absorption means 7 in that a linearmotor 8 causes absorber wedges 13 arranged opposite one another in pairsto be displaced transversely towards one another in the direction ofarrow A, so that depth-staggered scanning of volume elements of thetarget volume can be carried out in rapid succession.

For that purpose the rotors 31 of the linear motor 8 are mounted on airbearings and are supplied with compressed air by a compressed aircylinder 32. In order to move the absorber wedges 13 closer togetherstepwise, the motor currents are controlled by way of a stepper motorcontrol 35, which has one power stage.

In order to measure and add together the number of the ions until apredetermined ion beam dose per volume element of the target volume hasbeen reached and until the linear motor 8 can move the absorber wedges13 closer together by a further step, preferably by from 10 to 100 μm, apulse control having TTL pulses, the frequency of which is proportionalto the ion rate occurring (ions/sec), is controlled by means of theionisation chamber 16 by way of a current amplifier having a downstreamvoltage-frequency converter 33. As soon as a sufficient dose for avolume element has been reached, which corresponds to a specific TTLpulse count, the pulse control triggers the next step by a control pulseto the stepper motor control. That procedure is repeated until a depthscan has been completed.

FIG. 2 shows the principle of the overlaying of depth dose profilesdisplaced relative to one another. By means of the depth scanner, theindividual Bragg curves 36 to 44 are displaced by 4.3 mm in each casefrom one absorber position to the next. The height of the Bragg curvesis given by the number of particles that strike the target at thecorresponding absorber position. If the particle counts have beencalculated correctly in advance, the overlaying of the Bragg curvesyields the desired broadened Bragg peak, which corresponds to the extentof the target volume in depth. Accordingly, the depth of penetration isindicated along the abscissa in cm and the relative dose is indicatedalong the ordinate in %.

In that example, it can clearly be seen that when the Bragg curves areadded together, the target volume is irradiated with 100% relative dose,whilst the healthy tissue above it is subjected to less than 60% of theirradiation load, and the tissue beneath it only has to absorb wellbelow 20% of the radiation dose. The extent of the advantage of ion beamtherapy over X-ray therapy thus becomes clear.

FIG. 2 is merely intended to clarify the principle of overlaying. In areal depth scan, several thousand Bragg curves having much smallerspacings are overlaid. That enables virtually continuous movement of thedrives of the energy absorption means 7 and thus ensures uniformoverlaying so that conventional ripple filters are no longer necessarywhen the ion beam scanning system according to that embodiment of theinvention is used.

FIG. 3 shows the principle of the scanning of a target volume in columnsusing the embodiment according to FIG. 1 with ion beam 3 in the ion beamdirection 17 and penetrating depth of the ion beam 3 in the z-direction12. In that embodiment, the target volume is displaced mechanically inthe direction of the arrows X and Y, whilst the ion beam 3 of a rigidion beam guidance system retains its central direction. By the depthmodulation or depth scan caused by the energy absorption means, thevolume elements 9 of the target volume 5 are scanned in columns, theextent 1 of the broadened Bragg peak corresponding to the length of thecolumn or the depth of the target volume at that point by virtue ofsummation of the Bragg curves 36 to 43. As FIG. 3 clearly shows, healthytissue 10 can be sparred from irradiation to as great an extent aspossible, whilst the tumour tissue can assume a very wide variety ofshapes, which means that only the depth modulation has to follow theextent of the tumour tissue in one direction of coordinates, for examplealong the z-coordinate.

FIG. 4 is a perspective view of energy absorption means in the form of asingle absorber wedge. That absorber wedge is a single wedge of anabsorber wedge assembly, as shown in FIG. 6. The absorber wedge angle αis from 6 to 10° and in that example is set at 8.765°±0.01. The lengthof the absorber wedge is from 100 to 150 mm and in that example is setat 120±0.02 mm. The greatest thickness of the absorber wedge is from 15to 30 mm and in that example is set at 19±0.01 mm. The parallelipipedalend of the absorber wedge serves for stacking it in an assembly, asshown in FIG. 6. The dimensions of the parallelipipedal end are from15×15 mm² to 30×30 mm² with a length of from 40 to 60 mm, which, in thisexample, is 50 mm. The parallelipipedal cross-section is matched to thegreatest absorber wedge depth of 19 mm and in this example is 20×20 mm².A central hole 44 enables a plurality of individual wedges to becombined to form absorber wedge assemblies, as shown in FIG. 6.

FIG. 5 is a cross-section through energy absorption means comprising twoabsorber wedges 13 that can be moved towards one another in thedirection of arrow A or moved apart in the direction of arrow B. Whenthose two absorber wedges are moved apart, the result is a greaterspacing a between the inclined absorber wedge surfaces. Owing to theconverging centred ion beam 3, it becomes clear that the region of theion beam on the right-hand side of the beam has to pass through agreater thickness of absorber material than on the left-hand side of thebeam. Thus the depth of penetration into the target volume viewed overthe cross-section of the ion beam is different. In order to balance andminimize that, in FIG. 6 the cross-section of energy absorption means 7comprising absorber wedge assemblies 18 each comprising five absorberwedges 13 is shown. It can clearly be seen that the right-hand side ofthe convergent ion beam has to penetrate scarcely more absorber materialthan the left-hand side. Thus, depth modulation using absorber wedgeassemblies 18 is preferable to depth modulation using two absorberwedges 13, as shown in FIG. 5.

The absorber wedges 13 preferably consist of a plurality of plexiglasswedges. Placed one on top of the other as absorber wedge assemblies 18,they have the same effect as two absorber wedges 13 of FIG. 5 but with adifferent gradient. A large gradient is necessary in the case of twoabsorber wedges, as in FIG. 5, in order to obtain a sufficient absorbereffect with the obtainable acceleration of the absorber wedges 13. InFIG. 6, that large gradient is distributed over a plurality of absorberwedges having a small gradient.

The advantage of having a plurality of absorber wedges 13 placed on topof one another compared with two thick absorber wedges 13 is that thebeam is unavoidably scattered in the absorber wedges and, in the case ofthe two thick absorber wedges, some of the beam passes through morematerial than another part of the beam. The result is an undefinedenergy distribution of the beam, which it would be preferable to avoid.The absorber wedges 13 are secured to the linear motor 8 by aluminiumcorner pieces. The screw-fixings are precise and free of play.

Plexiglass as material for the absorber wedges 13 has the advantage thatit can be processed precisely and from a radiological point of view isvery similar to water. Accordingly, for a carbon ion beam a 1 cm thickplexiglass layer behaves in exactly the same manner as 1.15 cm thickwater layer. When 2×4 absorber wedges having an angle of gradient ofα=8.765° are used, a displacement by the motors by 1 mm increases thethickness of the absorber by a water-equivalent distance of 1.42 mm ofwater.

The ion beam scanning system according to the present invention makeshigh demands on the depth modulation system and on the mobility of theenergy absorption means 7.

The energy absorption means, or depth scanner, should produce with aheavy ion beam a smooth depth dose profile having a typical dimension offrom 2 to 15 cm in depth. For that purpose, there are used absorberwedges that can be so displaced that the distance through which the beampasses is altered. The depth scanner varies the absorber thickness in aprecisely defined manner during beam extraction. The heavy ion beams,which have passed through absorption settings of varying thickness, areoverlaid in the process and when added together produce the desireddepth dose profile.

In order that, in the overlaying, there are no departures from thepredetermined settings, in each setting of the absorber wedge thicknessa precisely defined amount of ion beam or number of particles muststrike the target. That means that when the absorber wedges remain in acertain position for too long or too short a period, there aredepartures from the desired depth dose profile. In order to avoid suchdepartures, the ion beam current must be measured as a function of timeand, depending on the number of ions measured, the absorber wedges areto be displaced continuously with high precision and varying step speed.

Since the ion beam current, especially in the case of heavy ionsaccelerated by an ion acceleration system, preferably a synchrotron, isnot constant, but exhibits very large fluctuations, that non-uniform ionbeam current results in the absorber wedges 13 also having to bedisplaced at very irregular speed and in some cases jerkily. That makesextremely high demands on the precision and dynamics of the drive andcontrol system for such energy absorption means 7, with the result thatthe following parameters are preferably to be observed:

Duration of extraction: 2-4 s Depth dose profile: 2-15 cm Maximum speed:1-2 m/s Acceleration: 20-30 m/s² Precision: 100-200 μm

Absorber wedge assemblies as shown in FIG. 6 are used for that purpose.

In order to carry out the displacement of the absorber wedges with asgreat synchronicity with the measured ion beam intensity as possible,the measuring and control electronics must react relatively quickly. Thereaction time, that is to say the sum of the delay time constants of theionisation chamber 16, the current amplifier 33 and the stepper motorcontrol 35, as shown in FIG. 1, is consequently less than 1 ms.

Lesser demands are made on the dynamics for the edge-delimination device20. FIG. 7 shows an edge-delimination device in the inoperative positionhaving displaceable shutter elements 21. In that example it consists ofsix individual rectangular tungsten plates 45 to 50, which can bedisplaced separately from their inoperative position shown in FIG. 7towards the center. For that purpose, the tungsten plates 45 to 50 arearranged offset in their height, so that they do not hinder one anotherwhen they are being pushed together. When the ion beam approaches theedge of a tumour tissue, or target volume 5, the corresponding edgeshutter 19, as shown in FIG. 8, can be displaced so enabling sharpdelimination of the edge for the relatively broad ion beam. Inprinciple, any edge of the tissue can be delimited even by threeheight-staggered shutters consisting of rectangular tungsten plates, butsix plates have the advantage of offering greater variation.

FIG. 9 is a schematic diagram of a linear motor 8 having an air bearing,to which motor an absorber assembly 18 of energy absorption means 7 canbe secured. Such a linear motor is a permanently excited two-phasereluctance motor and consists of two structural units, the stationarystator 51 and a mobile part, the rotor 52, having an air bearing.

A rotor comprises at least two magnet systems 53 and 54, each having apermanent magnet 55 as constant flux source. By means of a flow ofcurrent in the windings 56, it is possible to control the magnetic fluxin the limbs, which means that in one limb a flux is amplified and inthe associated other limb the flux is weakened. A dynamic effect in thedirection of movement is the result of a change in the energy of themagnetic field in the air gap between the stator 51 and the rotor 52. Ifboth windings of the magnet systems 53 and 54 are operated withlaterally displaced sinoidal currents, synchronized behaviour, or adefined position for each current relationship, is obtained. The rotorelement is arranged in the X-direction so that linear movements can beeffected. The stator element 51 consists of structured soft iron stripswhich are attached by adhesion to steel bodies, cast mineral material,natural hard rock or light glass fibres or hollow carbon fibre bodies.

In order to obtain aerostatic air bearing, each rotor 52 is providedwith air bearing jets. Owing to the electromagnetic dynamic effectbetween the rotor 52 and stator 51, the air bearing is under great biasso that at operating pressure the air gap is very constant. The servicelife is advantageously in principle unlimited since friction and wearand tear do not occur. Lubricants are also unnecessary. This facilitatesthe preferred use in the field of medicine.

For an embodiment of the ion beam scanning system according to theinvention, the linear motor was so designed that two motors move on thesame stator in opposite directions in the direction of arrow A, as shownin FIG. 1. For that purpose, the dimensions of the stator are 64 mm×64mm×1 m in length, of which only about 50 cm length are required for thedepth scanner. The stator also has a lamellar structure having a periodof 1.28 mm. The motors are mounted on air bearings and in principleoperate in the manner of magnetic levitation. The technical data of themotors is as follows:

Mass: 700 g Maximum holding force: 100 N Maximum speed: 2 m/s Maximumacceleration; 40 m/s² Positional accuracy: 10 μm Repeat accuracy: 3 μmGuiding accuracy: 5 μm Air pressure bearing; 3.5 bar

Advantageously, the stepper motor control 35, as shown in FIG. 1,position 35, is suitable for a depth scanner or a depth modulator. Thestepper motor control 35 has a simple effective interface that consistssubstantially of only 2 TL inputs 80, 81, which give a step command or adirection command. In the case of a step command, the motor currents ofthe power stage are so altered with each incoming TTL pulse at a firstinput 80 that the motors continue to move in opposite directions by 20μm in that example. For a direction command, the level (TTL) at thesecond input 81 determines the direction, that is to say movement apartor movement towards one another.

The ionisation chamber 16, as shown in FIG. 1, indicates the number ofparticles (heavy ions) extracted per second by the accelerator. Thatvalue is subject to large temporal variations and must therefore bemeasured in real time. For that purpose, there is used a parallel-platetransmission ionisation chamber having a gas distance of about 1 cm(nitrogen-CO₂ mixture: 80/20%), which is operated at 1600 V. The currentthat can be measured at the outlet of the chamber is proportional to thebeam current when the particle energy remains the same. At typical beamcurrents of the accelerator, the currents from the ionisation chamberare in the region of μA.

The response speed of the detector is limited by the drift time of theionised detector molecules (ion cores) in the ionisation chamber 16 andhas a delay constant of about 100 μs.

The measuring electronics in the current amplifier block 33 convert thecurrent from the ionisation chamber to a proportional frequency of TTLpulses. A voltage signal in the Volt range is produced from the currentsignal. TTL pulses are produced from the voltage signal byamplitude-frequency conversion, the frequency of which pulses isproportional to the voltage. 4 MHz correspond to about 10 V.

At a given amplification of the current amplifier 33 of FIG. 1, a TTLpulse accordingly corresponds to a specific charge produced in theionisation chamber, which was in turn produced by a specific particlecount. The number of TTL pulses produced is thus proportional to thenumber of heavy ions flying through the ionisation chamber 16. Theproportionality factor can be determined experimentally to an accuracyof ±3% and is dependent upon the particle energy and amplification.

The pulse control 34 in FIG. 1 is, in principle, a variable pulse rateconverter. The control system has a memory that can be divided into aplurality of regions. By way of the connected PC, a series of figurescan be entered for each of the individual regions before irradiation.The number of figures corresponds to the number of positions that thelinear motors are to adopt for a depth scan, which, in practice, will beseveral thousand positions per region.

When the irradiation is started, the pulse control counts the pulsescoming from the measuring electronics and for its part sends a pulse tothe stepper motor control 35 when the pulse count for the first positionhas been reached. The counter is then reset to 0 and counting is carriedout again until the pulse count for the second position has beenreached, whereupon a TTL pulse is emitted again. That procedure isrepeated until the final position has been reached. The pulse controlthen sends a stop signal to the accelerator, whereupon the beamextraction is terminated within a few μs.

Directly thereafter the next memory region having a different series offigures can be activated in order to carry out a different depth scan.The computer 60 and monitor 14 for control means is used to load theseries of figures into the pulse control. Moreover, the computer cancontrol the stepper motor control 35 directly by way of an index card.The motors can thus be moved automatically to their reference positionor start position before the actual depth scan.

FIG. 10 shows a second embodiment of the invention. The depth scanner 70corresponds to the depth of FIG. 1 in the manner of its construction andits operation.

However, in this preferred application, a gantry system is used, whichenables the ion beam 3 in a controlled ion beam 26 to be rotated about agantry axis of rotation 28. By means of the depth modulator or depthscanner 70, the target volume 5 can be scanned in columns and byrotating the gantry system by a few degrees of angle, the target volumecan be scanned literally. A particular advantage of that arrangement isthe possibility of arranging the target volume above the isocentre 29 ofthe gantry system 27. The inlet channel for the ion beam is thusdivergent in the upstream direction, which means that both the skin of apatient and also healthy tissue are subjected to less irradiationbecause the ion beam irradiation in the region above the target volume 5is distributed over a greater volume. By using the gantry systemaccording to FIG. 10, the target volume carrier 30 has to be moved inone direction only, preferably in the direction of the gantry axis ofrotation 28, as shown by the arrow directions C.

FIG. 11 is a schematic diagram of the arrangement of a target volume 5in the embodiment according to FIG. 10 above the isocentre 29. Thatschematic diagram shows clearly the direction of gantry rotation D inwhich the target volume 5 is depth-scanned in successive columns usingthe depth scanner 70. For each volume element 9, the number of ions ismeasured in the ionisation chamber 16 and, if necessary, a sharp edgecut-off can be adjusted near the edges of the tumour tissue by means ofthe edge-delimitation device 20. It can also be seen clearly that theirradiation of healthy tissue 10 above the tumour tissue is distributedover a relatively large volume so that the amount of radiation to whichhealthy tissue is subjected is reduced.

Whereas in the system shown in FIGS. 10 and 11 the target volume carrier30 has to be moved in the longitudinal direction in order to irradiateall of the target volume, FIG. 12 is a schematic diagram of a thirdembodiment of the invention in which no further mechanical movement ofthe target volume is necessary after alignment of the target volumerelative to the isocentre. In the embodiment according to FIG. 12showing gantry 4, instead of the target volume carrier, it is the ionbeam that is deflected in the beam guidance plane of the gantry systemby means of the deflection magnets 23, 24 and 25 of the ion beamguidance system in the gantry system, which does not require muchgreater outlay for the currents in the deflection magnets since it isnot necessary to redesign the deflection magnets because the ion beamwould be deflected in the direction of the dipole gap of the lastdeflection magnets. In that system the same depth scanner 70 is used asin the first and second embodiments of the invention according to FIG. 1and FIG. 10.

1. An ion beam scanning system having an ion source device, an ionaccelerator system that can be set to an acceleration of ions requiredto obtain a maximum depth of penetration, and an ion beam guidancesystem (1) comprising an ion beam outlet window (2) for a convergingcentred ion beam (3), and a mechanical alignment system (4) for a targetvolume (5) to be scanned, wherein the scanning system (6) comprisesenergy absorption means (7) that are arranged in the path of the ionbeam between the target volume (5) and the ion beam outlet window (2)transverse to the centre of the ion beam and comprises absorber wedges(13) that can be displaced transverse to the centre of the ion beam, ahigh-performance linear motor (8) for rapid driving of the absorberwedges (13) and beam-intensity-controlled depth-scanning with transversedisplacement of the energy absorption means (7), so that depth-staggeredscanning of volume elements (9) of a tumour tissue (11) can be carriedout in rapid succession, and an ionisation chamber (16) for measuringbeam intensity, which is arranged upstream of the energy absorptionmeans (7).
 2. The ion beam scanning system according to claim 1, whereinthe target volume (5) is the tumour tissue (11) surrounded by healthytissue (10), and wherein the depth of penetration (12) of the ion beam(3) is determined by energy of the ions in the ion beam (3) and adeepest region of the tumour tissue (11) can be reached by means of thevariable acceleration of the ions.
 3. The ion beam scanning systemaccording to claim 1, wherein the scanning system further comprises anelectronic control system (14, 34) for the linear drive of the absorberwedges (13) and includes an ionisation chamber (16) for measuring theparticle rate of the beam and moves the absorber wedges (13) closertogether by a step, when a predetermined particle count has beenreached, which particle count may be different for each depth step, soenabling depth-staggered scanning of volume elements (9) of the targetvolume (5).
 4. The ion scanning system according to claim 3, wherein thestep is from 10 μm to 100 μm.
 5. The ion beam scanning system accordingto claim 1, wherein the energy absorption means (7) comprise at leasttwo absorber wedges (13) that can be displaced in opposite directionstransverse to the centre of the ion beam.
 6. The ion beam scanningsystem according to claim 1, wherein the energy absorption means (7)comprise two absorber wedge assemblies (18) that can be oppositetransverse to the centre (17) of the ion beam.
 7. The ion scanningsystem according to claim 1, wherein the scanning system furthercomprises an edge-delimitation device (20) having displaceable shutterelements (21) between the target volume (5) and the energy absorptionmeans (7).
 8. The ion beam scanning according to claim 1, wherein thescanning system further comprises edge shutters (19) that can beadjusted separately in the manner of an iris diaphragm in order todelimit some of an edge of the ion beam (3) with respect to the targetvolume.
 9. The ion beam scanning system according to claim 1, furthercomprising a patient table (22) that carries the target volume (5) andthat can be displaced in a plane transverse to the ion beam (3) in twodirections of co-ordinates during an irradiation procedure.
 10. The ionbeam scanning system according to claim 1, further comprising a patienttable (22) that carries the target volume (5) and that can be displacedin a lateral direction transverse to the ion beam (3) during anirradiation procedure and has deflection magnets (23, 24, 25) thatdeflect the ion beam (3) from its central position at the outlet window(2) transverse to the lateral direction of the patient table (22). 11.The ion beam scanning system according to claim 1, wherein the intensityof the ion beam scanning is defined by a total number of ions thatstrike the volume element (9).
 12. The ion beam scanning systemaccording to claim 1, further comprising a patient table (22) thatcarries the target volume (5) and that can be displaced in a lateraldirection transverse to the ion beam (3) during irradiation, and agantry system (27) that can be rotated about a gantry area axis ofrotation transverse to the lateral direction of movement of the patienttable (22).
 13. The ion beam scanning system having the gantry system(27) for aligning the ion beam (3) with the target volume (5) accordingto claim 12, wherein the ion beam (3) is supplied to the gantry system(27) in the gantry axis of rotation (28) and can be aligned with atarget volume (5) by means of magneto-optics (23, 24, 25) at adjustableangles of from 0 to 360° in a plane orthogonal to the gantry axis ofrotation (28) so that the ion bean (3) intersects the gantry axis ofrotation (28) at an isocentre (29) of the gantry system (27), whereinthe gantry system (27) comprises a target volume carrier (30) that canbe displaced laterally in the direction of the gantry axis of rotation(28), the target volume (5) is arranged upstream of the isocentre (29)and energy absorption means (7), which are arranged radially upstream ofthe gantry system (27), define volume element scanning in a depthdirection, the gantry system (27) defines angular volume elementscanning in a lateral direction and the laterally displaceable targetvolume carrier (30) defines volume element scanning in a longitudinaldirection, and target volumes (5) of any shape can be scanned by volumeelement by a combination of the energy absorption means (7), the gantrysystem (27) and the target volume carrier (30).
 14. The ion beamscanning system having the gantry system according to claim 13 whereinthe target volume carrier (30) remains stationary during irradiation andthe deflection magnets (23, 24, 25) deflect the ion beam (3) in thegantry plane during irradiation.
 15. The ion beam scanning system havingthe gantry system according to claim 13, wherein the energy absorptionmeans (7) comprise absorber wedges (13) that can be displaced tangentialto a circle rotation of the gantry system (27).
 16. The ion beamscanning system having the gantry system according to claim 13, whereinthe energy absorption means (7) comprise at least two absorber wedges(13) that can be displaced in opposite directions tangential to a circleof rotation of the gantry system (27).
 17. The ion beam scanning systemhaving the gantry system according to claim 13, wherein the energyabsorption means (7) comprise absorber wedge assembles (18) that can bedisplaced in a radially staggered manner tangential to a circle ofrotation of the gantry system (27).
 18. An ion beam scanning systemhaving an ion source device, an ion accelerator system that can be setto an acceleration of ions required to obtain a maximum depth ofpenetration, and an ion beam guidance system (3), and a mechanicalalignment system (4) for a target volume (5) to be scanned, wherein thescanning system (6) comprises energy absorption means (7) that arearranged in the path of the ion beam between the target volume (5) andthe ion beam outlet window (2) transverse to the centre of the ion beamand comprises absorber wedges (13) that can be displaced transverse tothe centre of the ion beam, a high-performance linear motor (8) forrapid driving of the absorber wedges (13) and beam-intensity-controlleddepth-scanning with transverse displacement of the energy absorptionmeans (7), so that the depth-staggering scanning of volume elements (9)of a tumour tissue (11) can be carried out in rapid succession, and apatient table (22) that carries the target volume (5) and that can bedisplaced in a lateral direction transverse to the ion beam (3) duringirradiation, and a gantry system (27) that can be rotated about a gantryarea axis of rotation transverse to the lateral direction of movement ofthe patient table (22) and wherein the ion beam (3) is supplied to thegantry system (27) in the gantry axis of rotation (28) and can bealigned with a target volume (5) by means of magneto-optics (23, 24, 25)at adjustable angles of from 0 to 360° in a plane orthogonal to thegantry axis of rotation (28) so that the ion bean (3) intersects thegantry axis of rotation (28) at an isocentre (29) of the gantry system(27), wherein the gantry system (27) comprises a target volume carrier(30) that can be displaced laterally in the direction of the gantry axisof rotation (28), the target volume (5) is arranged upstream of theisocentre (29) and energy absorption means (7), which are arrangedradially upstream of the gantry system (27), define volume elementscanning in a depth direction, the gantry system (27) defines angularvolume element scanning in a lateral direction and the laterallydisplaceable target volume carrier (30) defines volume elements scanningin a longitudinal direction, and target volumes (5) of any shape can bescanned by volume element by a combination of the energy absorptionmeans (7), the gantry system (27) and the target volume carrier (30) andwherein a central region of the target volume (5) is arranged upstreamof the isocentre (29) by at least one fifth of a radius of the gantrysystem, so that the target volume (5) does not lie in the isocentre(29).
 19. A method of ion beam scanning using an ion source device, anion accelerator system and an ion beam guidance system (1) comprising anion beam outlet window (2) for a converging centered ion beam (3), and amechanical alignment system (4) for scanning a target volume (5), thescanning system (6) comprising energy absorption means (7) and anionisation chamber (16), which is arranged upstream of the energyabsorption means (7), comprising the steps of: setting the ionaccelerator system to an acceleration of the ions required to obtain amaximum depth of penetration (12), detecting ion beam intensity,traversing displacement of the energy absorption means (7) of variablethickness for depth modulation of the ion bean (3), summing radiationions of a volume element (9) of the target volume (5) up to apredetermined radiation dose, altering the depth of penetration of theion beam (3) by means of transverse displacement of the energyabsorption means (7) when the predetermined radiation dose of the volumeelement (9) has been reached in order to irradiate a next upstreamvolume element.
 20. The method of operating an ion beam scanning systemaccording to claim 19 using a gantry system (27), wherein a targetvolume carrier (30) is aligned before irradiation and remains stationaryduring irradiation and the ion beam (3) is deflected in the gantry planeby means of the last gantry deflecting magnets (23, 24, 25) in order tocarry out volume element scanning in a longitudinal direction.
 21. Themethod according to claim 19, wherein an electronic control system (14)for a linear drive of the absorber wedges (13) measures a particle rateof the beam by means of the ionisation chamber (16) and moves theabsorber wedges (13) closer together by a step, after a predeterminedparticle rate has been reached, which particle rate may be different foreach depth step, so that a depth-staggered scanning of volume elements(9) of the target volume (5) is effected.
 22. The method according toclaim 21, wherein the step is from 10 μm to 100 μm.
 23. A method of ionbeam scanning using an ion source device, an ion accelerator system andan ion beam guidance system (1) comprising an ion beam outlet window (2)for a converging centered ion beam (3), and a mechanical alignmentsystem (4) for scanning a target volume (5), the scanning system (6)comprising energy absorption means (7) and an ionisation chamber (16),which is arranged upstream of the energy absorption means (7),comprising the steps of: setting the ion accelerator system to anacceleration of the ions required to obtain a maximum depth ofpenetration (12), detecting ion beam intensity, traversing displacementof the energy absorption means (7) of variable thickness for depthmodulation of the ion beam (3), summing radiation ions of a volumeelement (9) of the target volume (5) up to a predetermined radiationdose, altering the depth of penetration of the ion beam (3) by means oftransverse displacement of the energy absorption means (7) when thepredetermined radiation doses of the volume element (9) has been reachedin order to irradiate the next upstream volume element wherein theintensity is adjusted to from 10⁶ to 10⁸ absorbed ions per volume unitduring scanning of the target volume (5).
 24. The method according toclaim 19, wherein the scanning of the volume of the target volume (5)progresses continuously.
 25. The method according to claim 19, whereinthe scanning of the volume of the target volume (5) in a depth directionis effected by columns.
 26. The method according to claim 19, whereinthe scanning of the volume of the target volume (5) proceeds stepwise.27. The method according to claim 19, wherein the scanning of the targetvolume (5) is carried out continuously in a depth direction and stepwisein a lateral and longitudinal directions.
 28. The method according toclaim 19, wherein the scanning of the volume of the target volume (5) iscarried out continuously in a depth direction and in a lateral directionand stepwise in a longitudinal direction.
 29. The method of operating inion beam scanning system according to claim 19 using a gantry system(27) for aligning the ion beam (3) with the target volume (5), whereinthe ion beam (3) is supplied to the gantry system (27) in an axis ofrotation and is aligned with the target volume (5) by means ofmagneto-optics (23, 24, 25) at adjustable angles of from 0 to 360° in aplane orthogonal to the gantry axis of rotation (28), so that the ionbeam (3) intersects the gantry axis of rotation (28) at an isocentre(29) of the gantry displaced laterally in the direction of the gantryaxis of rotation (28), comprising the steps of: arranging the targetvolume (5) upstream of an isocentre (29), scanning the volume element ina depth direction by means of energy absorption means (7) arrangedradially upstream of the gantry system (27), scanning the volume elementin a lateral direction by altering the angle of rotation of the gantrysystem (27), and scanning of the volume element in a longitudinaldirection by lateral displacement of the target volume carrier (30). 30.An ion beam scanning system having an ion source device, an ionaccelerator system that can be set to an acceleration of ions requiredto obtain a maximum depth of penetration, and an ion beam guidancesystem (1) comprising an ion beam outlet window (2) for a convergingcentred ion beam (3), and a mechanical alignment system (4) for a targetvolume (5) to be scanned, wherein the scanning system (6) comprisesenergy absorption means (7) that are arranged in the path of the ionbeam between the target volume (5) and the ion beam outlet window (2)transverse to the centre of the ion beam and comprises absorber wedges(13) that can be displaced transverse to the centre of the ion beam, ahigh-performance linear motor (8) for rapid driving of the absorberwedges (13) and beam-intensity-controlled depth-scanning with transversedisplacement of the energy absorption means (7), so that depth-staggeredscanning of volume elements (9) of a tumour tissue (11) can be carriedout in rapid succession and wherein the linear motor comprises rotors(31) mounted on air bearings.